1. Field of the Invention
This invention relates generally to electronic identification and recognition systems and more specifically to such systems wherein it is not necessary to make physical contact between the identification section and the reader section.
2. Description of the Prior Art
There are various electronic identification and recognition systems wherein the identifying device is portable. Sometimes the identifying device is referred to as a "card", "tag", "key" or the like. The recognition station or reader station is prepared to recognize idetifying devices of predetermined characteristics when such identifying device is brought within the proximity of the reader station. Inductive coupling takes place between the two devices when they are proximate to one another and it is not necessary to bring the device into physical contact. Though the identifying device may contain a battery, it usually receives its energy from the reader section.
Uses for such systems may include identification of person having identifying devices in their possession. For example, the identifying device may take the shape of a credit card and have an electronic circuit embedded therein for radiating signals of identifying intelligence. An individual possessing the card may position it adjacent a door that the individual desires to enter. A recognition device may be arranged to control the door latch and if it recognizes radiated signals of certain predetermined intelligence, the door latch responds to the reader when the individual places the "card" proximate to the reader. Other uses for such systems include having the identifier in the forn of a tag attached to a vehicle to be identified. As the vehicle passes a certain location, its identification is recognized and recorded. Also, in production lines, garments or other items may carry identifying tags so that they can be appropriately processed as they are recognized along various points in the production processes.
The prior art includes various patents and patent applications by the present inventor. The patents include U.S. Pat No. 3,752,960 for "Electronic Identification and Recognition System," issued Aug. 14, 1973; U.S. Pat. No. 3,816,708 for "Electronic Recognition and Identification System," issued June 11, 1974; U.S. . No. 3,816,709 for "Electronic Identification and REcognition System," issued June 11, 1974; U.S. Pat. No. 4,223,830 for "Identification System," issued Sept, 23, 1980; U.S. Pat. No. 4,236,068 for "Personal Identification and Signalling System," issued Nov. 25, 1980; U.S. Pat. No. 4,384,288 for "Portable Radio Frequency Emitting Identifier," issued May 17, 1983; U.S. Pat. No. 4,388,524 for "Electronic Identification and Recognition with Code Changeable Reactance," issued June 14, 1983; U.S. Pat. No. 4,459,474 for "Identification System with Separation and Direction Capability and Improved Noise Reduction"; U.S. Pat. No. 4,473,825 for "Electronic Identification System with Power Input-Output Interlock and Increased Capabilities", issued Mar. 30, 1984; U.S. Pat. No. 4,546,241 for "Identification System with Synchronous Modulation of Return Signal", issued Dec. 10, 1982; U.S. Pat. No. 4,580,041 for "Electronic Proximity Identification System with Low Power Identifier, Simplified"; U.S. Pat. No. 4,600,829 for "Electronic Proximity Identification and Recognition System with Isolated Two-Way Coupling", issued on July 15, 1986; and U.S. patent application Ser. No. 06/637,546 for "Identification System with Vector Phase Angle Detection", filed on Aug. 3, 1984.
In the typical prior art systems, the reader section emits flux vertically from a single horizontal coil antenna. In the case of a vehicle identification system, the reader coil antenna is buried beneath the road surface. The identifier section has its own identifier antenna. In operation, the identifier section is positioned such that the identifier antenna is parallel to the reader coil antenna such that the identifier section antenna is penetrated by vertical flux lines from the reader antenna. Such systems of the prior art have some problems when the identifier section is attached to an object. For example, if the identifier section is located next to a metal vehicle body or undercarriage or ship's surface, there is interference with the vertical path of the flux. A large portion of the flux which rises vertically from the reader antenna cannot penetrate the identifier antenna resulting in loss of range and reliability.
FIG. 1 is an end cross-sectional view of the prior art electronic proximity identification system used with a vehicle and is referred to by the general reference number 10. A reader antenna 20 is buried under a road surface 30. The antenna 20 is positioned parallel with a surface 30. The circle with a dot represents current flowing through antenna 20 in a direction out of the paper, and the circle with the X inside represents current flowing through antenna 20 in a direction into the paper. Using the right-hand rule, the flux field generated has flow lines 40.
A vehicle 60 with tires 62 and 64 passes over road surface 30 with an identifier section 70 attached beneath the underside of vehicle 60. The flux lines 40 flow up through an identifier antenna 72. However, the bottom of the vehicle 60 blocks some of the flux lines 40. This reduces range and reliability of the identifier section.
One solution to the problem is to position the identifier antenna far enough away from the interfering metal surface so that more of the vertical flux lines can penetrate the identifier antenna. Locating the identifier antenna farther away from the object increases the likelihood that it will be damaged, and also makes it more expensive to mount the identifier section to the object.
Another problem with the prior art identification systems occurs at higher frequencies. High frequencies are desirable for minimizing the number of turns required in the identifier antenna. At higher frequencies, the wavelengths of the radio signal can be such that standing wave effects occur. This can interfere with the transmission of identification data from the identifier section to the reader section.
Still another problem with prior art identification systems involves radio interference. In electronic identification systems, it is necessary that the Federal Communications Commission give its approval, since there is a risk that such identification systems will cause interference to radio frequency communications. To meet FCC requirements, identification systems operate at either very low powers, or on certain allowed frequencies, or with antennas of minimum long distance radiation. The present invention avoids harmful far field radiation through self neutralizing antenna design, in a manner to be described below.